What I meant was : we can't guess rationally what should happen like in real world, because causality is not the only force at work. Fate can interfere anytime, and since we are not in capacity to guess what he will decides, we can't affirm "This will happen" or "this can't happen" in advance. From our point of view, anything can happen, even a hobbit slaying a dark lord. But in the end there is only one possible road for the events, wich can be called fate, or Tolkien's will in the books, and finally Jackson's vision in the films. Hum... I'm not sure I made myself any clearer...